Transistor short circuit protection

ABSTRACT

A short circuit detection circuit includes a current terminal, a sense resistor, an amplifier, and a resistor-capacitor ladder. The sense resistor is coupled to the current terminal, and is configured to develop a sense voltage proportional to a current through the current terminal. The amplifier is coupled to the sense resistor, and is configured to generate a scaled current proportional to the sense voltage. The resistor-capacitor ladder is coupled to the amplifier, and is configured to generate a measurement voltage that represents a surface temperature rise due to the current through the current terminal.

BACKGROUND

Transistors, such as metal oxide semiconductor (MOS) transistors, areused as power switches in various applications. In some applications,transistors are stacked in series between two power supply rails. Thisarrangement is referred to as half-bridge configuration. If a shortcircuit develops between the two power rails through the stackedtransistors, one or both of the transistors may be damaged.

SUMMARY

A short-circuit detection circuit that identifies a short-circuitcondition based on estimated die temperature is described herein. In oneexample, a short circuit detection circuit includes a current terminal,a sense resistor, an amplifier, and a resistor-capacitor ladder circuit.The sense resistor includes a first terminal coupled to the currentterminal, and a second terminal. The amplifier includes a first inputcoupled to the first terminal of the sense resistor, a second inputcoupled to the second terminal of the sense resistor, and an output. Theresistor-capacitor ladder circuit includes an input coupled to theoutput of the amplifier.

In another example, a short circuit detection circuit includes a currentterminal, a sense resistor, an amplifier, and a resistor-capacitorladder. The sense resistor is coupled to the current terminal, and isconfigured to develop a sense voltage proportional to a current throughthe current terminal. The amplifier is coupled to the sense resistor,and is configured to generate a scaled current proportional to the sensevoltage. The resistor-capacitor ladder is coupled to the amplifier, andis configured to generate a measurement voltage that represents asurface temperature rise due to the current through the currentterminal.

In a further example, a circuit includes a switching transistor, a sensetransistor, a sense resistor, an amplifier, and a resistor-capacitorladder circuit. The switching transistor includes a current terminal,and a control terminal. The sense transistor includes a first currentterminal, a second current terminal, and a control terminal. The firstcurrent terminal is coupled to the current terminal of the switchingtransistor. The control terminal is coupled to the control terminal ofthe switching transistor. The sense resistor includes a first terminaland a second terminal. The first terminal of the sense resistor iscoupled to the second current terminal of the sense transistor. Theamplifier includes a first input, a second input, and an output. Thefirst input is coupled to the first terminal of the sense resistor. Thesecond input is coupled to the second input of the sense resistor. Theresistor-capacitor ladder circuit includes an input coupled to theoutput of the amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for an example short circuit detection circuitthat detects a short-circuit condition based on estimation of surfacetemperature.

FIG. 2 is a schematic diagram for an example resistor-capacitor (R-C)ladder circuit that models a surface portion of switching transistorsemiconductor material;

FIG. 3 is a schematic diagram for an example amplifier and currentsources of the short circuit detection circuit of FIG. 1 .

DETAILED DESCRIPTION

Protecting low resistance transistors against shorts has becomeincreasing difficult. Short circuit currents may be high (e.g., in therange of 500 amperes) and to prevent damage transistors should beprotected across all slew rates. Overcurrent detection circuitry may beprovided to monitor current flow through transistors, and triggerprotection when an overcurrent condition is detected. Conventionalovercurrent detection circuitry measures current flow through atransistor, and compares the measured current to a threshold to identifyan overcurrent condition. Because of transients generated duringtransistor switching, conventional overcurrent detection may be blankedout (ignored) until switching is complete and the transistor output hassettled. After the transistor output has settled, if the measuredcurrent exceeds the threshold, an overcurrent condition is deemed toexist. However, the time (blanking time) during which overcurrentdetection is disabled may be too long or too short given process andslew rate variation. If the blanking time is too long, and anovercurrent or short circuit condition exists, the transistor may bedamaged. If the blanking time is too short, false detection of anovercurrent condition may occur.

The short circuit detection circuit described herein detects a shortcircuit condition during transistor switching (during blanking time) byestimating the surface temperature of the switching transistor. Thesurface temperature of the transistor increases with current flow(energy dissipation across the transistor), and if the estimated surfacetemperature exceeds a threshold value, then a short circuit condition isdeemed to exist. The short circuit detection circuit measures currentflow through a switching transistor via a sense transistor. A highbandwidth amplifier converts the current flowing through the sensetransistor to a scaled current in a low voltage domain. The scaledcurrent is applied to a resistor-capacitor (R-C) ladder circuit thatmodels a surface portion of the semiconductor material of the switchingtransistor. When the voltage across the R-C ladder circuit exceeds athreshold, indicating a die surface temperature greater than apredetermined temperature, a short circuit condition is deemed to exist,and the switching transistor may be turned off to prevent damage.

FIG. 1 is a block diagram for an example circuit 100 that includes shortcircuit detection based on estimated surface temperature of a switchingtransistor. The circuit 100 includes a short circuit detection circuit102, a high-side transistor 104, and a low-side transistor 106. Thehigh-side transistor 104 and the low-side transistor 106 are connectedin a half-bridge configuration. The high-side transistor 104 and thelow-side transistor 106 may be switching transistors of a DC-DCconverter, an inverter, a power factor correction circuit, or any othercircuit that implements stacked transistors.

The high-side transistor 104 and the low-side transistor 106 areillustrated as n-type field effect transistors. The high-side transistor104 may be a p-type field effect transistor in some implementations ofthe circuit 100. The high-side transistor 104 and the low-sidetransistor 106 may be gallium nitride (GaN) high electron mobilitytransistors (HEMTs). In FIG. 1 , a drain of the high-side transistor 104is coupled to a power supply terminal, and source of the high-sidetransistor 104 is coupled to a drain of the low-side transistor 106. Asource of the low-side transistor 106 is coupled to ground. The gate ofthe high-side transistor 104 and the gate of the low-side transistor 106are coupled to a control circuit or a control terminal (not shown) thatprovides a high-side control signal to the high-side transistor 104 andthe low-side control signal for turning the high-side transistor 104 andthe low-side transistor 106 on or off.

The short circuit detection circuit 102 is coupled to a switching node(a current node 107) formed at the connection of the high-sidetransistor 104 (e.g., the source of the high-side transistor 104) to thelow-side transistor 106 (the drain of the low-side transistor 106). Theshort circuit detection circuit 102 estimates the temperature of thesurface semiconductor material of the low-side transistor 106 todetermine whether a short circuit exists at the current node 107. Theshort circuit detection circuit 102 includes a sense transistor 108, asense resistor 110, a resistor 112, an amplifier 114, a current source118, a current source 120, an R-C ladder circuit 122, and a comparator124. The sense transistor 108 is a scaled-down replica of the low-sidetransistor 106, and passes a sense current that is a scaled-down replica(a predetermined fraction) of the current flowing in the current node107 and the low-side transistor 106. The drain 108D (a current terminal)of the sense transistor 108 is coupled to the current node 107, and thegate 108G (a control terminal) of the sense transistor 108 is coupled tothe gate (a control terminal) of the low-side transistor 106.

The sense transistor 108 is coupled to the sense resistor 110, and thesense current flowing through the sense transistor 108 also flows in thesense resistor 110 to develop a sense voltage across the sense resistor110. A terminal 110A of the sense resistor 110 is coupled to the sourceterminal 108S (a current terminal) of the sense transistor 108, andterminal 1106 of the sense resistor 110 is coupled to ground.

The sense resistor 110 is coupled to the amplifier 114. The amplifier114 scales the voltage across the amplifier 114 to generate a scaledcurrent that is proportional to the sense voltage. A first input of theamplifier 114 is coupled to the terminal 110A of the sense resistor 110,and a second input 1146 of the amplifier 114 is coupled to ground viathe resistor 112. A terminal 112B of the resistor 112 is coupled toground, and a terminal 112A of the resistor 112 is coupled to the input1146 of the amplifier 114.

The output signal of the amplifier 114 controls the current source 118and the current source 120. The output 114C of the amplifier 114 iscoupled to the control input 1186 of the current source 118 and thecontrol input 120B of the current source 120. The current source 118produces, based on the output signal of the amplifier 114, a scaledcurrent (a feedback current) that is fed back to the input 1146 of theamplifier 114. The output 118C of the current source 118 is coupled tothe input 114B of the amplifier 114.

The current source 120 produces, based on the output signal of theamplifier 114, a scaled current that provided to the R-C ladder circuit122. While the high-side transistor 104, the low-side transistor 106,and the sense transistor 108 may operate in a high voltage domain (e.g.,hundreds of volts), the amplifier 114, the current sources 118 and 120,the R-C ladder circuit 122, and the comparator 124 operate in a lowvoltage domain (e.g., 5 volts or less). Accordingly, the output currentsof the current source 118 and the current source 120 are provided in thelow voltage domain. The power input 118A of the current source 118 andthe power input 120A of the current source 120 are coupled a powersupply terminal 116 (a low voltage power supply terminal).

The comparator 124 is coupled to the R-C ladder circuit 122 and comparesthe voltage dropped across the R-C ladder circuit 122 to a thresholdvoltage. The voltage across the R-C ladder circuit 122 represents anestimate of the surface temperature of the semiconductor material of thelow-side transistor 106. Thus, the voltage across the R-C ladder circuit122 increases with the current flowing in the sense transistor 108 asthe surface temperature of the semiconductor material of the low-sidetransistor 106 increases with the current flowing in the sensetransistor 108. An input 124A of the comparator 124 is coupled to theoutput 120C of the current source 120 and the input 122A of the R-Cladder circuit 122 for receipt of an estimated temperature signal (ameasurement voltage) developed across the R-C ladder circuit 122. Aninput 124B of the comparator 124 is coupled to a reference voltagesource 126 for receipt of a reference voltage representing a shortcircuit temperature threshold. The reference voltage may represent asurface temperature of approximately 200° Celsius in someimplementations of the short circuit detection circuit 102. An output124C of the comparator 124 provides a signal indicating detection of ashort circuit based on estimated die surface temperature.

FIG. 2 is a schematic diagram for an example R-C ladder circuit 122 thatmodels a surface portion of the semiconductor material of the low-sidetransistor 106. The R-C ladder circuit 122 includes capacitors that formthe rungs of the R-C ladder circuit 122, and resistors that areconnected in series form the rail of the R-C ladder circuit 122. Theresistors emulate the thermal conductivity of the semiconductormaterial, and the capacitors emulate the thermal capacitance of thesemiconductor material. The implementation of the R-C ladder circuit 122shown in FIG. 2 includes ten rungs. The ten rungs include capacitors222, 224, 226, 228, 230, 232, 234, 236, 238, and 240. Each capacitor mayhave a capacitance of about 3.2 picofarads in some implementations ofthe R-C ladder circuit 122. The rail of the R-C ladder circuit 122includes resistors 202, 204, 206, 208, 210, 212, 214, 216, 218, 220, and221. The resistor 202 may have a resistance of about 2.6 kilohms, andthe resistors 204-220 may each have a value of about 5.2 kilohms in someimplementations of the R-C ladder circuit 122. The resistor 202 at oneend of the rail is coupled to the input 122A, and the resistor 221 atthe other end of the rail is coupled to the input 122B. The input 122Bis coupled to ground.

A bottom plate terminal of each capacitor is coupled to the input 122Bof the R-C ladder circuit 122. A top plate terminal of each capacitor iscoupled to at least one of the resistors of the rail. For example, thetop plate terminal of the capacitor 222 is coupled to the resistor 202and the resistor 204, the top plate terminal of the capacitor 224 iscoupled to the resistor 204 and the resistor 206, etc.

In the R-C ladder circuit 122, each rung emulates the thermalcapacitance of approximately one micron of depth of the surfacesemiconductor material of the low-side transistor 106. Thus, given theten rungs shown in FIG. 2 , the R-C ladder circuit 122 emulates a10-micron depth of semiconductor material. In short circuit conditionsof 200 nanoseconds or less, a heatwave in GaN does not diffuse more than10 microns. Some implementations of the R-C ladder circuit 122 mayinclude a different number of rungs to represent a different depth ofsemiconductor material.

FIG. 3 is a schematic diagram for examples of the amplifier 114 andcurrent sources 118 and 120 of the short circuit detection circuit 102.The amplifier 114 includes a differential input pair 301 and a biascircuit 305. The differential input pair 301 has common-mode gateconfiguration, and includes the transistor 302 and the transistor 304.The output 114C of the amplifier 114 is coupled to the drain of thetransistor 304. The source of the transistor 304 is coupled to the input1146. The source of the transistor 302 is coupled to the input 114A viathe resistor 313.

The bias circuit 305 sources current to the differential input pair 301,and includes the transistor 306, the transistor 308, the transistor 310,and the transistor 312. The source of the transistor 310 and the sourceof the transistor 312 are coupled to the power supply terminal 116. Thedrain of the transistor 310 is coupled to the source of the transistor306, and the drain of the transistor 312 is coupled to the source of thetransistor 308. The drain of the transistor 306 is coupled to the drainof the transistor 302, and the drain of the transistor 308 is coupled tothe drain of the transistor 304.

The current source 118 includes a transistor 314, a transistor 316, anda resistor 318. The source of the transistor 314 is coupled to the powersupply terminal 116. The gate of the transistor 314 is pulled down suchthat the transistor 314 is always on. The drain of the transistor 314 iscoupled to the source of the transistor 316 via the resistor 318. Thegate of the transistor 316 is coupled to the output 114C of theamplifier 114, so that current flow through the transistor 316 iscontrolled by the output signal of the amplifier 114. The drain of thetransistor 316 is coupled to the input 114B of the amplifier 114.

The current source 120 includes selectable current sources 319, 321, and323. Some implementations of the current source 120 may include adifferent number of selectable current sources (more or less than threeselectable current sources). The current sources 319, 321, and 323 maybe individually enabled or disabled via a current selection value. Thecurrent selection value may be set at manufacture based on the currentrating of the low-side transistor 106. The current source 319 includes atransistor 320, a transistor 322, and a resistor 324. The source of thetransistor 320 is coupled to the power supply terminal 116. The gate ofthe transistor 320 is controlled by the current selection value toenable or disable the current source 319. The drain of the transistor320 is coupled to the source of the transistor 322 via the resistor 324.The gate of the transistor 322 is coupled to the output 114C of theamplifier 114, so that current flow through the transistor 322 iscontrolled by the output signal of the amplifier 114. The drain of thetransistor 322 is coupled to the output 120C for driving the R-C laddercircuit 122.

The current source 321 includes a transistor 326, a transistor 328, anda resistor 330. The source of the transistor 326 is coupled to the powersupply terminal 116. The gate of the transistor 326 is controlled by thecurrent selection value to enable or disable the current source 321. Thedrain of the transistor 326 is coupled to the source of the transistor328 via the resistor 330. The gate of the transistor 328 is coupled tothe output 114C of the amplifier 114, so that current flow through thetransistor 328 is controlled by the output signal of the amplifier 114.The drain of the transistor 328 is coupled to the output 120C fordriving the R-C ladder circuit 122.

The current source 323 includes a transistor 332, a transistor 334, anda resistor 336. The source of the transistor 332 is coupled to the powersupply terminal 116. The gate of the transistor 332 is controlled by thecurrent selection value to enable or disable the current source 323. Thedrain of the transistor 332 is coupled to the source of the transistor334 via the resistor 336. The gate of the transistor 334 is coupled tothe output 114C of the amplifier 114, so that current flow through thetransistor 334 is controlled by the output signal of the amplifier 114.The drain of the transistor 334 is coupled to the output 120C fordriving the R-C ladder circuit 122.

In FIG. 3 , the R-C ladder circuit 122 also includes a reset input 122Cfor receiving a reset signal. The reset signal, when active, dischargesthe capacitors of the R-C ladder circuit 122 in preparation for a nexttemperature estimation. The reset signal activates the transistor 346 topull the input 122A to ground to discharge the capacitors of the R-Cladder circuit 122. The reset signal provided at the reset input 122Cmay close switches coupled in parallel with one or more of thecapacitors of the R-C ladder circuit 122 to discharge the capacitors.

Short circuit detection as described herein with respect to the low-sidetransistor 106 may also be applied to the high-side transistor 104 bycoupling an instance of the short circuit detection circuit 102 acrossthe high-side transistor 104. For example, the drain 108D of the sensetransistor 108 may be coupled to the drain of the high-side transistor104, the gate 108G of the sense transistor may be coupled to the gate ofthe high-side transistor 104, and the terminal 110B of the senseresistor 110 may be coupled to the source of the high-side transistor104.

In this description, the term “couple” may cover connections,communications or signal paths that enable a functional relationshipconsistent with this description. For example, if device A generates asignal to control device B to perform an action, then: (a) in a firstexample, device A is directly coupled to device B; or (b) in a secondexample, device A is indirectly coupled to device B through interveningcomponent C if intervening component C does not substantially alter thefunctional relationship between device A and device B, so device B iscontrolled by device A via the control signal generated by device A.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A short circuit detection circuit, comprising: acurrent terminal; a sense resistor including: a first terminal coupledto the current terminal; and a second terminal; an amplifier including:a first input coupled to the first terminal of the sense resistor; asecond input coupled to the second terminal of the sense resistor; andan output; and a resistor-capacitor ladder circuit comprising: an inputcoupled to the output of the amplifier.
 2. The short circuit detectioncircuit of claim 1, wherein the resistor-capacitor ladder circuitincludes: a first rung including a first capacitor; a second rungincluding a second capacitor; a first resistor coupled to the firstcapacitor and the second capacitor; and a second resistor coupled to thefirst capacitor and the first resistor.
 3. The short circuit detectioncircuit of claim 1, further comprising a comparator including: a firstinput coupled to the input of the resistor-capacitor ladder circuit; anda second input coupled to a reference voltage source terminal.
 4. Theshort circuit detection circuit of claim 1, further comprising: a sensetransistor including: a first current terminal coupled to the currentterminal; and a second current terminal coupled to the first terminal ofthe sense resistor.
 5. The short circuit detection circuit of claim 1,further comprising: a first resistor including: a first terminal coupledto the second input of the amplifier; and a first terminal coupled to aground terminal.
 6. The short circuit detection circuit of claim 1,further comprising: a current source comprising: a power input coupledto a power supply terminal; an output coupled to the second input of theamplifier; and a control input coupled to the output of the amplifier.7. The short circuit detection circuit of claim 1, further comprising: acurrent source comprising: a power input coupled to a power supplyterminal; an output coupled to the input of the resistor-capacitorladder circuit; and a control input coupled to the output of theamplifier.
 8. A short circuit detection circuit, comprising: a currentterminal; a sense resistor coupled to the current terminal, andconfigured to develop a sense voltage proportional to a current throughthe current terminal; an amplifier coupled to the sense resistor, andconfigured to generate a scaled current proportional to the sensevoltage; and a resistor-capacitor ladder coupled to the amplifier, andconfigured to generate a measurement voltage that represents a surfacetemperature rise due to the current through the current terminal.
 9. Theshort circuit detection circuit of claim 8, further comprising: acomparator coupled to the resistor-capacitor ladder and configured tocompare the measurement voltage to a reference voltage.
 10. The shortcircuit detection circuit of claim 9, wherein the comparator isconfigured to generate a signal indicating detection of a short circuitbased on the measurement voltage exceeding the reference voltage. 11.The short circuit detection circuit of claim 8, wherein theresistor-capacitor ladder includes: a rail including resistors coupledin series; and capacitors forming rungs coupled to the rail.
 12. Theshort circuit detection circuit of claim 11, wherein: the resistorsemulate a thermal conductivity of a semiconductor material; and thecapacitors emulate a thermal capacitance of a surface of thesemiconductor material.
 13. The short circuit detection circuit of claim8, further comprising: a sense transistor coupled to the currentterminal and the sense resistor, and configured to pass a sense currentto the sense resistor, wherein the sense current is a fraction of thecurrent through the current terminal.
 14. The short circuit detectioncircuit of claim 8, further comprising: a current source coupled to theamplifier, and configured to provide a feedback current at an input ofthe amplifier.
 15. The short circuit detection circuit of claim 8,further comprising: a current source coupled to the amplifier and theresistor-capacitor ladder, and configured to provide the scaled currentto the resistor-capacitor ladder.
 16. A circuit, comprising: a switchingtransistor including: a current terminal; and a control terminal; asense transistor including: a first current terminal coupled to thecurrent terminal of the switching transistor; a second current terminal;and a control terminal coupled to the control terminal of the switchingtransistor; a sense resistor, including: a first terminal coupled to thesecond current terminal of the sense transistor; and a second terminal;an amplifier including: a first input coupled to the first terminal ofthe sense resistor; a second input coupled to the second terminal of thesense resistor; and an output; and a resistor-capacitor ladder circuitcomprising: an input coupled to the output of the amplifier.
 17. Thecircuit of claim 16, further comprising further comprising a comparatorincluding: a first input coupled to the input of the resistor-capacitorladder; and a second input coupled to a reference voltage sourceterminal.
 18. The circuit of claim 16, further comprising: a firstcurrent source comprising: a power input coupled to a power supplyterminal; an output coupled to the second input of the amplifier; and acontrol input coupled to the output of the amplifier.
 19. The circuit ofclaim 18, further comprising: a second current source comprising: apower input coupled to the power supply terminal; an output coupled tothe input of the resistor-capacitor ladder circuit; and a control inputcoupled to the output of the amplifier.
 20. The circuit of claim 16,wherein the resistor-capacitor ladder circuit includes: rungs havingrespective capacitors; and a rail having resistors connected in seriesand coupled to respective ones of the rungs.